Phytohaemagglutinin skin-swelling test in scarlet rosefinch males: low-quality birds respond more strongly

Abstract

The phytohaemagglutinin (PHA) skin-swelling test is one of the most widely used methods for cell-mediated immunity measurement in immunoecology. Although several studies have investigated the condition-dependent traits associated with the magnitude of cutaneous inflammatory response to PHA, the results concerning signalling of the responsiveness through ornamental traits are still controversial. This is especially true for carotenoid-based feather ornamentation in birds. We therefore examined the linkage between several condition-dependent traits, including the red ornamental coloration of the plumage, and the magnitude of the PHA-induced immune response in scarlet rosefinch, Carpodacus erythrinus, males. Our results show two important aspects of the PHA-induced inflammation in this species. First, histological analysis showed that the swelling response was dependent on basophil activity. Second, the magnitude of the response (increase in patagium thickness) was associated with individual size, carotenoid-based ornamental coloration and a ptilochronological marker of feather growth at the time of moulting (mean growth bar width), thus mirroring the long-term quality of the individual. The positive linkage between the individual size or mean growth bar width and the PHA response suggests an association between the magnitude of the response and individual metabolic rate. However, as the magnitude of the response was also related negatively to ornament saturation and positively to ornament lightness, our results indicate stronger responsiveness in inferior males. Highly ornamented, healthier individuals recruited fewer basophils into the inflamed tissue causing less intense swelling. To our knowledge, this study is the first to show a negative association between carotenoid-based plumage coloration and the magnitude of the PHA-induced immune response.